The present exemplary embodiment relates to a molten metal pump having gas and/or flux introduction capabilities. It finds particular application in conjunction with an overflow transfer style of pump, and will be described with particular reference thereto.
Pumps for pumping molten metal are used in furnaces in the production of metal articles. Common functions of pumps are circulation of molten metal in the furnace or transfer of molten metal to remote locations. The present description is focused on molten metal pumps for transferring metal from one location to another. It finds particular relevance to systems where molten metal is elevated from a furnace bath into a launder system.
Currently, many metal die casting facilities employ a main hearth containing the majority of the molten metal. Solid bars of metal may be periodically melted in the main hearth. A transfer pump can be located in a well adjacent the main hearth. The transfer pump draws molten metal from the well and transfers it into a ladle or conduit, and from there, to die casters that form the metal articles. The present disclosure relates to pumps used to transfer molten metal from a furnace to a die casting machine, ingot mold, or the like.
In aluminum foundries where castings are made using either high pressure die casting or gravity die casting techniques, ladles are often used for transporting pre-measured quantities of liquid metal from a holding furnace to a casting machine and then pouring the liquid metal into a receptacle of the casting machine. The ladle can be filled by using a molten metal transfer pump to move metal from the furnace to the ladle. One particular molten metal transfer pump described herein is referred to as an overflow transfer pump. For example, the overflow transfer pump in U.S. Publication No. 2013/0101424, herein incorporated by reference, is suitable.
Molten metals such as aluminum may include oxide and/or nitride debris that have a negative effect on the solidification of the particular alloy. A fluxing process is one methodology used to remove such impurities. Flux injection is the process of introducing a powdered or granulated salt mixture such as chloride and/or fluoride into the molten aluminum. Traditionally, the salt flux has been introduced by simply depositing the flux in a ladle before or during molten metal addition and/or using a rotary apparatus for introduction of the flux in the ladle or downstream from the ladle.
An exemplary rotary apparatus includes a central hollow shaft attached to a rotor inserted into a pool of molten aluminum and rotated such that the salt flux travels down the hollow shaft and is dispersed within the molten aluminum through apertures in the rotor. This style of flux injection device has proven problematic as failure to control the flow rate of the purge gas used to keep the molten metal out of the shaft during insertion into the bath can cause molten metal splash. Similarly, the high flow process gas used after insertion can cause molten metal splash. Conversely, a disruption in the gas feed line (e.g., kink or bend) has the cascade effect of allowing the flux injecting shaft/rotor assembly to become clogged with flux and/or molten metal ingress. Moreover, since the shaft/rotor assembly of the traditional device is disposed below the molten metal line, improper handling can result in hardening of metal therein, causing the device to become inoperative.
Flux addition by simple deposit in the ladle may not achieve a homogenous dispersion of the flux throughout the molten metal. Furthermore, use of a rotary fluxing apparatus in the ladle or at a downstream location introduces an undesirable time delay to the casting process.
The melted or liquefied form of aluminum also attracts the formation and absorption of hydrogen within the molten aluminum. Hydrogen evolves as porosity during the solidification of aluminum alloys and is detrimental to the mechanical properties of the solid alloy. Degassing is an effective way of reducing hydrogen caused porosity. One example of degassing involves introducing an inert gas such as argon or nitrogen into the molten aluminum to collect hydrogen and non-metallic inclusions. The gas bubbles to the surface with the hydrogen and other inclusions. Similar to fluxing, this process has been historically performed in the ladle and/or at a downstream processing station. Accordingly, undesirable time delays result.
The present disclosure is directed to a system for introducing flux and/or gas to molten metal in a highly efficient manner. Moreover, the present system is believed to provide comparable flux introduction results while improving efficiency and safety. The present disclosure is directed to an improved, more efficient introduction of flux and/or inert gas at the molten metal transfer pump, before filling of the ladle. Moreover, it has been found that a more homogenous mixture of flux within the molten metal can be achieved with introduction of small quantities of flux over time into a moving stream of metal. Similarly, it has been found that the quality of the metal can be improved by the introduction of an inert gas early in the transfer process of the metal from furnace to casting apparatus. Exemplary locations for flux/gas injection may include the column of an overflow transfer pump or the second chamber of divided chamber overflow transfer apparatus or the launder into which molten metal is directed.